Using a forward genetic approach we have identified much of the lipopolysaccharide (IPS) signaling apparatus, and clarified how several components interact with one another. Our work began with the positional cloning of the Lps locus, which revealed TLR4 as the core signaling element of the LPS receptor. Subsequently, during the previous period of funding, we used ENU to create variant alleles of genes encoding many of the proteins that are required for the cytokine response to LPS. Cytokines orchestrate the systemic inflammatory response that we know as sepsis. But where the outcome of sepsis is concerned, the production of cytokines is only part of the story. The sensitivity of host tissues and organ systems to the cytokine response is a major determinant of survival during sepsis, and little is known about how sensitivity is determined. Using random mutagenesis, we have found that Kir6.1, a subunit of a widely-expressed ATP- sensitive inwardly rectifying potassium channel (KATP) encoded by the KcnJ8 gene, is absolutely required for mice to survive infection by mouse cytomegalovirus, or challenge with minute doses of LPS, poly I:C, or type II interferon (IFNv). On the other hand, the mutants show wildtype resistance to tumor necrosis factor (TNF), a MyD88-dependent cytokine well known to mediate many LPS effects. The relevant KATP is an essential component of a homeostatic response that allows the host to survive IFNy induction, and therefore challenge with LPS, poly I:C,or diverse microbes. In fact, we posit that KATP exists chiefly to enable the host to cope with innate immune responses that inevitably occur in the course of life. We also believe that there may be many similar examples of proteins that "stabilize" the host during infection, and these proteins may be the principal determinants of outcome during a systemic infection. Using random germline mutagenesis, we propose to identify other genes encoding the proteins required for mice to survive low-dose LPS challenge, and to establish their relationship to one another, probing the mechanism of protection in each case. The means by which KATP offers protection will be analyzed in detail, using a suppressor screen, proteomic tools, and focused experiments intended to identify "post-immune" proteins and metabolic changes that lead to death. This work will enrich our understanding of how and why infections are sometimes lethal.